Optical modules

ABSTRACT

Provided is an optical module. The optical module includes: an optical bench having a first trench of a first depth and a second trench of a second depth that is lower than the first depth; a lens in the first trench of the optical bench; at least one semiconductor chip in the second trench of the optical bench; and a flexible printed circuit board covering an upper surface of the optical bench except for the first and second trenches, wherein the optical bench is a metal optical bench or a silicon optical bench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 14/724,147, filed May28, 2015, which is a division of U.S. application Ser. No. 14/294,603,filed Jun. 3, 2014 (now U.S. Pat. No. 9,069,146, issued Jun. 30, 2015),which is a division of U.S. application Ser. No. 13/154,859, filed Jun.7, 2011 (now U.S. Pat. No. 8,774,568, issued Jul. 8, 2014). Further,this application claims priority to Korean Application No.10-2010-0115478, filed Nov. 19, 2010. The disclosure of these U.S. andKorean applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention disclosed herein relates to optical modules, andmore particularly, to optical modules for transmitting a high frequencysignal.

In order to transmit and process a high frequency signal in a opticaltransmission or/and optical reception module(s) of more than about 10Gbps, a ceramic submount is used inside a metal package and a FlexiblePrinted Circuit Board (FPCB) is used outside the metal package. In theceramic submount and the FPCB, an electrode of a CoPlanar Waveguide(CPW) form is used. Also, in order to transmit a high frequency signalwithout reflection, a termination matching resistor is integrated on theceramic submount with a thin-film type and in order to preventdistortion phenomenon of a high frequency signal due to resonances, viaholes that electrically connect an upper ground electrode and a lowerground electrode are formed. A process for forming the thin-film typetermination matching resistor and the via holes on the ceramic submountrequires high costs so that it accounts for a significant portion of thetotal cost for fabricating an optical module. Moreover, the forming ofthe thin-film type termination matching resistor of a high quality onthe ceramic submount or the forming of the elaborate via holes requireshigh technology.

Furthermore, a ceramic feed-through is formed in a metallic package bodyaccording to an optical module structure using the ceramic submount sothat a high frequency transmission line of the ceramic submount in themetallic package body is connected to a high frequency transmission lineformed on the ceramic feed-through through bonding wires, and a highfrequency transmission line of the FPCB at the outside of the metallicpackage is connected to a high frequency transmission line formed on theceramic feed-through through soldering. Generally, if a high frequencysignal is connected by bonding wires through a plurality of steps, it isreflected because of characteristic impedance mismatch, so that highfrequency signal characteristics of an optical module are seriouslydeteriorated.

SUMMARY OF THE INVENTION

The present invention provides an optical module for minimizing mismatchof characteristic impedance and improving a high frequency signalcharacteristic by suppressing high frequency resonances.

Embodiments of the present invention provide optical modules including:an optical bench having a first trench of a first depth and a secondtrench of a second depth that is lower than the first depth; a lens inthe first trench of the optical bench; at least one semiconductor chipin the second trench of the optical bench; and a flexible printedcircuit board covering an upper surface of the optical bench except forthe first and second trenches, wherein the optical bench is a metaloptical bench or a silicon optical bench.

In some embodiments, the optical modules may further include athermo-electric cooler unit contacting on an entire lower surface of theoptical bench, which faces the flexible printed circuit board.

In other embodiments, the optical modules may further include a metalpackage component surrounding the optical bench having the lens and thesemiconductor chip and the flexible printed circuit board to protectthem from an external environment.

In still other embodiments, the metal package component may have a slit;and the flexible printed circuit board may extend to an external of themetal package component through the slit.

In even other embodiments, a ceramic feed-through may be provided in themetal package component; the ceramic feed-through may be electricallyconnected to an external flexible printed circuit board outside themetal package component; and the flexible printed circuit board may beelectrically connected to the ceramic feed-through inside the metalpackage component.

In yet other embodiments, the flexible printed circuit board may beelectrically connected to the ceramic feed-through through a ribbon wireor a bonding wire; and the external flexible printed circuit board maybe electrically connected to the ceramic feed-through through soldering.

In further embodiments, the metal package component may include areceptacle for connecting to an external ferrule, with a window adjacentto the lens.

In still further embodiments, the optical modules may further include amatching resistor on the flexible printed circuit board.

In even further embodiments, the semiconductor chip may include at leastone of an electro-absorption modulated laser, a capacitor, a photodiode,a laser diode, or a thermistor.

In yet further embodiments, the flexible printed circuit board mayinclude: a conductive line; a lower ground line; and an insulation layerbetween the conductive line and the lower ground line, wherein theconductive line includes a signal transmission line, an upper groundline, and an electrode line.

In yet further embodiments, the flexible printed circuit board may havevia holes for electrically connecting the upper ground line with thelower ground line; and the upper ground line and the lower ground linemay be electrically connected to each other through a conductivematerial filling at least a portion of the via holes.

In yet further embodiments, the lower ground line of the flexibleprinted circuit board may be electrically connected to the opticalbench.

In yet further embodiments, the signal transmission line and the upperground line may be a coplanar waveguide or a microstrip line.

In yet further embodiments, the insulation layer may have a dielectricconstant of about 2 to about 4 and may have a dissipation factor ofabout 0.001 to about 0.05.

In yet further embodiments, the insulation layer may include a polyimideor Teflon.

In yet further embodiments, the insulation layer may have a thickness ofabout 20 μm to about 80 μm.

In yet further embodiments, the metal optical bench may include one ofcopper-tungsten, copper, kovar, an aluminum alloy, or a combinationthereof.

In yet further embodiments, the silicon optical bench may include: asilicon substrate; and a gold plating layer on the silicon substrate.

In yet further embodiments, the lens may be a lens fixed with a metalhousing or a lens of a bare chip shape.

In other embodiments of the present invention, optical modules include:a thermo-electric cooler unit; a first optical bench with a lens on thethermo-electric cooler unit; a second optical bench with a trench,contacting on the first optical bench except for a region with the lens;at least one semiconductor chip in the trench of the second opticalbench; a flexible printed circuit board covering an upper surface of thesecond optical bench except for the trench; and a metal packagecomponent surrounding the thermo-electric cooler unit, the first opticalbench with the lens, the second optical bench with the semiconductorchip, and the flexible printed circuit board to protect them from anexternal environment, wherein the first and second optical benches are ametal optical bench or a silicon optical bench.

In still other embodiments of the present invention, optical modulesinclude: a thermo-electric cooler unit; a first optical bench with alens on the thermo-electric cooler unit; a second optical bench with atrench on the thermo-electric cooler unit, being spaced from the firstoptical bench; at least one semiconductor chip in the trench of thesecond optical bench; a flexible printed circuit board covering an uppersurface of the second optical bench except for the trench; and a metalpackage component surrounding the thermo-electric cooler unit, the firstoptical bench with the lens, the second optical bench with thesemiconductor chip, and the flexible printed circuit board to protectthem from an external environment, wherein the first and second opticalbenches are a metal optical bench or a silicon optical bench.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention, and are incorporated in andconstitute a part of this specification. The drawings illustrateexemplary embodiments of the present invention and, together with thedescription, serve to explain principles of the present invention. Inthe drawings:

FIG. 1 is a sectional view of a schematic configuration illustrating anoptical module according to an embodiment of the present invention;

FIG. 2 is a plan view of a schematic configuration illustrating aportion A of FIG. 1 to describe an optical module according toembodiments of the present invention;

FIG. 3 is a sectional view taken along the line I-I′ of FIG. 2 todescribe an optical module according to embodiments of the presentinvention;

FIG. 4 is a sectional view of a schematic configuration illustrating anoptical module according to another embodiment of the present invention;

FIG. 5 is a sectional view of a schematic configuration illustrating anoptical module according to further another embodiment of the presentinvention; and

FIG. 6 is a sectional view of a schematic configuration illustrating anoptical module according to further another embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. Advantagesand features of the present invention, and implementation methodsthereof will be clarified through following embodiments described withreference to the accompanying drawings. The present invention may,however, be embodied in different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art. Further, the present invention is only defined by scopes ofclaims. Like reference numerals refer to like elements throughout.

In the following description, the technical terms are used only forexplaining specific embodiments while not limiting the presentinvention. The terms of a singular form may include plural forms unlessreferred to the contrary. The meaning of “include,” “comprise,”“including,” or “comprising,” specifies a property, a region, a fixednumber, a step, a process, an element and/or a component but does notexclude other properties, regions, fixed numbers, steps, processes,elements and/or components. It will be understood that when an elementsuch as a layer, film, region, or substrate is referred to as being “on”another element, it can be directly on the other element or interveningelements may also be present.

Additionally, the embodiment in the detailed description will bedescribed with sectional views as ideal exemplary views of the presentinvention. In the figures, the dimensions of layers and regions areexaggerated for clarity of illustration. Accordingly, shapes of theexemplary views may be modified according to manufacturing techniquesand/or allowable errors. Therefore, the embodiments of the presentinvention are not limited to the specific shape illustrated in theexemplary views, but may include other shapes that may be createdaccording to manufacturing processes. Areas exemplified in the drawingshave general properties, and are used to illustrate a specific shape ofa semiconductor package region. Thus, this should not be construed aslimited to the scope of the present invention.

FIG. 1 is a sectional view of a schematic configuration illustrating anoptical module according to an embodiment of the present invention. FIG.2 is a plan view of a schematic configuration illustrating a portion Aof FIG. 1 to describe an optical module according to embodiments of thepresent invention. FIG. 3 is a sectional view taken along the line I-I′of FIG. 2 to describe an optical module according to embodiments of thepresent invention.

Referring to FIGS. 1 through 3, the optical module 100A includes aThermo-Electric Cooler (TEC) 110, an optical bench 120, a lens 140,semiconductor chips 152, 154, 156, and 158, a Flexible Printed CircuitBoard (FPCB) 130, a matching resistor 150, and a metal package component100.

The micro-sized TEC 110 is provided in the metal package component 100.The TEC 110 may be provided in the metal package component 100 by asupport plate 104 and a support pillar 106. Typically, the support plate104 and the support pillar 106 may be included in the TEC 110. That is,the support plate 104, the support pillar 106, and the TEC 110 may beone thermo-electric device structure. This may be used to maintain auniform internal temperature of the optical module 100A and also maysecure stable operation of the optical module 100A.

The optical bench 120 is provided on the TEC 110. The optical bench 120may have a first trench 122 and a second trench 124. The first trench122 may have a first depth and the second trench 124 may have a seconddepth lower than the first depth. The optical bench 120 may use ametallic material having excellent thermal conductivity and electricalconductivity and small thermal expansion. The optical bench 120 may be aMetal Optical Bench (MOB) or a Silicon Optical Bench (SiOB). The MOB mayinclude at least one of copper-tungsten (CuW), copper, kovar, an Alalloy, or a combination thereof. A gold plating layer may beadditionally provided on the surface of the MOB to further improveelectrical conductivity. The SiOB may include a silicon substrate and agold plating layer on the silicon substrate. The silicon substrate mayhave excellent thermal conductivity and the gold plating layer mayimprove electrical conductivity.

The lens 140 and the semiconductor chips 152, 154, 156, and 158 may beprovided in the first trench 122 and the second trench 124 of theoptical bench 120, respectively. The second depth of the second trench124 may be substantially identical to or less than the height of each ofthe semiconductor chips 152, 154, 156, and 158. The semiconductor chips152, 154, 156, and 158 may include an Electro-absorption Modulated Laser(EML) 152, a PhotoDiode (PD) 154, a capacitor 156, a thermistor 158, ora laser diode (LD).

A pipe 160 having a window 170 for transmitting/receiving light and areceptacle 165 coupled to the pipe 160 to connect to an external ferrule180 may be provided at a portion of the metal package component 100adjacent to the lens 140. The window 170 may include sapphire. Anisolator 190 may be provided between the ferrule 180 and the window 170.

The FPCB 130 having a signal transmission line 136 s as a form of aCoPlanar Waveguide (CPW) or a MicroStrip Line (MSL) may be extend fromthe external of the metal package component 100 into the inside of themetal package component 100 having the semiconductor chips 152, 154,156, and 158 through a slit 102 of the metal package component 100. TheFPCB 130 may cover the upper surface of the optical bench 120 except thefirst and second trenches 122 and 124 in the metal package component100. That is, a form of the FPCB 130 on the optical bench 120 may be afabricated structure without the first and second trenches 122 and 124.The FPCB 130 extending to the outside of the metal package component 100may be connected to an external Printed Circuit Board (PCB).

The semiconductor chips 152, 154, 156, and 158 may be fixed at theinside of the second trench 124 by using solder paste or silver pastehaving excellent thermal conductivity as a medium. The optical bench 120may perform a thermal conductivity function and also perform a groundfunction of a high frequency signal, simultaneously. Moreover, thethermistor 158 needs to be disposed around the other semiconductor chips152, 154, or 156 in order to achieve an accurate operation of the TEC110, so that it is disposed in the second trench 124 together with theother semiconductor chips 152, 154, or 156. As a result, heat occurringat the other semiconductor chips 152, 154, and 156 may be accuratelydetected.

The FPCB 130 may be attached on the upper surface of the optical bench120 using silver paste having excellent electrical conductivity.Accordingly, a lower ground line 132 of the FPCB 130 and the opticalbench 120 may be formed to be electrically connected with the sameground.

The FPCB 130 may consist of conductive lines 136 d, 136 g, and 136 s, aninsulation layer 134, and the lower ground lines 132. The conductivelines 136 d, 136 g, and 136 s of the FPCB 130 may include a signaltransmission line 136 s, upper ground lines 136 g, and electrode lines136 d. The signal transmission line 136 s and the upper ground lines 136g of the conductive lines 136 d, 136 g, and 136 s of the FPCB 130 mayhave a form of the CPW or the micro strip line.

The insulation layer 134 of the FPCB 130 has a dielectric constant ofabout 2 to about 4 and a very low dissipation factor of about 0.001 toabout 0.05. The insulation layer 134 of the FPCB 130 may includepolyimide or Teflon appropriate for high frequency signal transmission.The insulation layer 134 of the FPCB 130 may have a thickness of about20 μm to about 80 μm. Preferably, the insulation layer 134 of the FPCB130 may have a thickness of about 50 μm. Moreover, the insulation layer134 of the FPCB 130 may be formed of a material having the absorptioncharacteristic with the low moisture absorption of less than about 3% inorder to guarantee reliability.

Input/output pads (I/O pads) 137 i and 137 o for electrical connectionwith the semiconductor chips 152, 154, 156, and 158 integrated into thesecond trench of the optical bench 120 may be respectively provided onthe tops of ends of the conductive lines 136 d, 136 g, and 136 s of theFPCB 130. A gold plating layer may be additionally provided on the I/Opads 137 i and 137 o to improve bonding property. The output pads 137 oof the electrode lines 136 d of the FPCB 130 may be electricallyconnected to electrode pads 112 for the TEC 110 through bonding wires139 and the output pads 137 o of the signal transmission line 136 s andthe electrode lines 136 d of the FPCB 130 may be electrically connectedto the semiconductor chips 152, 154, 156, and 158 through bonding wires139. In particular, the output pad 137 o of the signal transmission line136 s of the FPCB 130 may be connected to electrode pad 153 for the EML152 through ribbon wire.

Since the semiconductor chips 152, 154, 156, and 158 are integrated onthe second trench 124 of the optical bench 120 and the FPCB 130 isprovided around the second trench 124, electrical connection portionsand connection paths between the semiconductor chips 152, 154, 156, and158 and the FPCB 130 may be minimized. Accordingly, loss of a highfrequency signal and impedance mismatch may be minimized.

The signal transmission line 136 s and the upper ground lines 136 g ofthe FPCB 130 may be a form of a Grounded CPW (GCPW) or a micro stripline. That is, the upper ground lines 136 g and the lower ground lines132 of the FPCB 130 are connected to each other to constitute the GCPW.If the GCPW is provided, via holes 138 may be further provided tosuppress resonance phenomenon by equalizing electric potential of theupper ground lines 136 g to that of the lower ground lines 132 of theGCPW. The via holes 138 may have a diameter of about 100 μm to about 250μm and a distance between the respectively adjacent via holes 138 may beabout 500 μm to about 900 μm. The via holes 138 may be easily formedthrough a tool drilling or laser drilling method. The upper ground lines136 g and the lower ground lines 132 of the GCPW may be electricallyconnected to each other through a conductive material filling at least aportion of the via holes 138.

The lens 140 for aligning and focusing the light emitted from the EML152 into an optical fiber is integrated into the inside of the metalpackage component 100. The lens 140 may be fixed at the bottom of thefirst trench 122. The lens 140 may be a lens fixed with metal housing ora lens of a bare chip shape. If the lens 140 is the square lens of thebare chip shape, it may be fixed at the bottom of the first trench 122of the optical bench 120 through epoxy or solder. Moreover, if the lensof the bare chip form is fixed at a Steel Us Stainless (SUS) ring, thelens 140 may be fixed at the bottom of the first trench 122 of theoptical bench 120 through a laser welding method. At this point, the SUSring outside the lens 140 and the surface of the optical bench 120 maybe fixed through laser welding. Especially, if the lens 140 is fixed atthe optical bench 120 through laser welding, the optical bench 120formed of excellent laser welding materials may be used. The opticalbench 120 may be the MOB using materials such as CuW, copper, and kovar.

The matching resistor 150 for termination without distortion of a highfrequency signal may be provided in the FPCB 130 through soldering orflip-chip bonding. The matching resistor 150 may have a Surface MountDevice (SMD) applicable to a typical PCB. The matching resistor 150 mayhave a size of about 200 μm to about 600 μm as parasitic components suchas parasitic capacitance or/and parasitic inductance are smaller in ahigh frequency. The matching resistor 150 may be electrically connectedto the EML 152 through the bonding wire 139 connecting electrode pad 137r for matching resistor 150 with electrode pad 153 for the EML 152.

Hereinafter, referring to FIGS. 4 through 6, optical modules accordingto other embodiments of the present invention will be described. FIGS. 4through 6 are sectional views of a schematic configuration of eachoptical module according to the embodiments of the present invention.Like elements refer to like numerical references throughout.

The difference between the optical module 100B according to anotherembodiment, described with reference to FIG. 4, and the optical module100A according to the above embodiment is that a configuration of theoptical bench is different.

The optical module 100B includes a TEC 110, first and second opticalbenches 120 a and 120 b, a lens 140, semiconductor chips 152 and 154,and 156 and 158 of FIG. 2, an FPCB 130, the matching resistor 150 ofFIG. 3, and a metal package component 100.

The lens 140 may be provided on the first optical bench 120 a. Thesecond optical bench 120 b having the trench 124 may be provided on thefirst optical bench 120 a except for a region having the lens 140.Inside the trench 124 of the second optical bench 120 b, thesemiconductor chips 152, 154, 156, and 158 may be provided. A method ofproviding the first optical bench 120 a and the second optical bench 120b may be possible by a junction between the gold plating layers on thesurface of the optical bench described in the above-mentionedembodiment. Additionally, the method of providing the second opticalbench 120 b on the first optical bench 120 a may be possible by ajunction applying silver paste between the gold plating layers of thefirst and second optical benches 120 a and 120 b.

The difference between the optical module 100C according to anotherembodiment, described with reference to FIG. 5, and the optical module100A according to the above embodiment is that a configuration of theoptical bench is different.

The optical module 100C includes a TEC 110, first and second opticalbenches 120 c and 120 d, a lens 140, semiconductor chips 152 and 154,and 156 and 158 of FIG. 2, an FPCB 130, the matching resistor 150 ofFIG. 3, and a metal package component 100.

The lens 140 may be provided on the first optical bench 120 c. Thesecond optical bench 120 d spaced from the first optical bench 120 cwith a trench 124 on the TEC 110 may be provided. The semiconductorchips 152, 154,156 and 158 may be provided in the trench 124 of thesecond optical bench 120 d.

The difference between the optical module 100D according to anotherembodiment, described with reference to FIG. 6, and the optical module100A according to the above embodiment is that the PCB 130 is providedonly in the metal package component 100.

The optical module 100D includes a TEC 110, a lens 140, semiconductorchips 152 and 154, and 156 and 158 of FIG. 2, an FPCB 130, the matchingresistor 150 of FIG. 3, and a metal package component 100.

A ceramic feed-through 200 may be provided in the metal packagecomponent 100. The FPCB 130 may be electrically connected to an externalFPCB 130 e through the ceramic feed-through 200 of the metal packagecomponent 100. That is, the FPCB 130 is connected in the metal packagecomponent 100 through pads 137 eo for the ceramic feed-through 200 andthe external FPCB 130 e is electrically connected to the ceramicfeed-through 200 outside the metal package component 100. The input pads137 i of the FPCB 130 and the pads 173 eo for the ceramic feed-through200 may be electrically connected to each other through a bonding wire139. The external FPCB 130 e may electrically contact the ceramicfeed-through 200 through soldering. That is, the FPCB 130 and theexternal FPCB 130 e are electrically connected to each other by usingthe ceramic feed-through 200 as a medium.

As mentioned above, an optical module according to embodiments of thepresent invention is electrically connected to FPCB for high frequencysignal transmission by using an MOB or a SiOB with an easy trenchformation so that mismatch of characteristic impedance may be minimizedand high frequency resonance may be suppressed. Accordingly, a highfrequency signal characteristic of an optical module may be improved.Moreover, characteristics of more than about 10 Gbps (especially, about25 Gbps) may be improved. Furthermore, compared to a typical opticalmodule using a ceramic submount, an optical module of the presentinvention may be technically easily fabricated with a lower cost. As aresult, an optical module of low cost and high quality may be provided.

Furthermore, since the optical module according to embodiments of thepresent invention uses an MOB having a higher thermal conductivity thana typical ceramic submount, so that it may have improved heatdissipation characteristics and less power consumption. Accordingly, ahighly reliable optical module may be provided.

Moreover, since the optical module according to embodiments of thepresent invention connects an MOB or a SiOB with a FPCB electrically, amatching resistor may be easily formed on the FPCB, unlike difficult andexpensive manufacturing processes for forming a matching resistor fortermination of a high frequency signal on a typical ceramic submount.Accordingly, an optical module of a low price and excellent quality maybe provided.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe true spirit and scope of the present invention. Thus, to the maximumextent allowed by law, the scope of the present invention is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. An optical module comprising: an optical benchhaving a first trench of a first depth and a second trench of a seconddepth that is lower than the first depth; a lens in the first trench ofthe optical bench; at least one semiconductor chip in the second trenchof the optical bench; a flexible printed circuit board covering an uppersurface of the optical bench except for the first and second trenches;and a metal package component surrounding the optical bench having thelens and the semiconductor chip and the flexible printed circuit boardto protect them from an external environment, wherein the optical benchis a metal optical bench or a silicon optical bench, wherein a ceramicfeed-through is provided in the metal package component; the ceramicfeed-through is electrically connected to an external flexible printedcircuit board outside the metal package component; and the flexibleprinted circuit board is electrically connected to the ceramicfeed-through inside the metal package component.
 2. The optical moduleof claim 1, wherein the flexible printed circuit board is electricallyconnected to the ceramic feed-through through a ribbon wire or a bondingwire; and the external flexible printed circuit board is electricallyconnected to the ceramic feed-through through soldering.
 3. The opticalmodule of claim 1, wherein the metal package component comprises areceptacle for connecting to an external ferrule, with a window adjacentto the lens.
 4. The optical module of claim 1, further comprising amatching resistor on the flexible printed circuit board.
 5. The opticalmodule of claim 1, wherein the semiconductor chip comprises at least oneof an electro-absorption modulated laser, a capacitor, a photodiode, alaser diode, or a thermistor.
 6. The optical module of claim 1, whereinthe flexible printed circuit board comprises: a conductive line; a lowerground line; and an insulation layer between the conductive line and thelower ground line, wherein the conductive line comprises a signaltransmission line, an upper ground line, and an electrode line.
 7. Theoptical module of claim 6, wherein the flexible printed circuit boardhas via holes for electrically connecting the upper ground line with thelower ground line; and the upper ground line and the lower ground lineare electrically connected to each other through a conductive materialfilling at least a portion of the via holes.
 8. The optical module ofclaim 6, wherein the lower ground line of the flexible printed circuitboard is electrically connected to the optical bench.
 9. The opticalmodule of claim 6, wherein the signal transmission line and the upperground line are a coplanar waveguide or a microstrip line.
 10. Theoptical module of claim 1, wherein the metal optical bench comprises oneof copper-tungsten, copper, kovar, an aluminum alloy, or a combinationthereof.
 11. The optical module of claim 1, wherein the silicon opticalbench comprises: a silicon substrate; and a gold plating layer on thesilicon substrate.
 12. The optical module of claim 1, wherein the lensis a lens fixed with a metal housing or a lens of a bare chip shape.